The invention relates to a method of manufacturing a magnetic field sensor having a stacked structure which comprises:
a substrate; PA1 an exchange-biasing layer comprising nickel oxide; PA1 a magnetic layer which is exchange-biased with the exchange-biasing layer, whereby at least the exchange-biasing layer is provided by sputter deposition. PA1 as magnetic heads, which can be used to decrypt the magnetic flux emanating from a recording medium in the form of a magnetic tape, disc or card; PA1 in compasses, for detecting the terrestrial magnetic field, e.g. in automotive, aviation, maritime or personal navigation systems; PA1 as field sensors in medical scanners, and as replacements for Hall probes in various other applications; PA1 as memory cells in Magnetic Random-Access Memories (MRAMs).
Magnetic field sensors of this type may be employed inter alia:
The term "nickel oxide" as employed throughout this text should be interpreted as referring to any stoichiometric or non-stoichiometric compound of nickel and oxygen. Although the symbol "NiO" will frequently be used in this context, this symbol should be viewed as encompassing compounds of the form NiO.sub.1.+-..delta., in which .delta. is a positive fraction. The term "substrate" should be interpreted as referring to any body of material on which the exchange-biasing layer can be provided; such a body may therefore be composite (e.g. in the case of a glass plate coated with a layer of another material). It should be explicitly noted that the magnetic layer may be located above or below the exchange-biasing layer. Furthermore, the term "exchange-biasing" should be interpreted as encompassing both horizontal exchange-biasing (also referred to as longitudinal biasing) and vertical exchange-biasing (also referred to as perpendicular or transverse biasing).
A method as specified in the opening paragraph is known from European Patent Application EP 594 243, with the exception that the exchange-biasing layer therein described is comprised of an iron-manganese alloy (FeMn) instead of nickel oxide. In the described sensor, which exploits so-called spin-valve magneto-resistance effects, a first and a second layer of permalloy are magnetically coupled across an intervening layer of Cu, and the first permalloy layer is also exchange-biased with an adjacent FeMn layer. As a result of this exchange-biasing, the magnetization of the first permalloy layer is "pinned" in position; consequently, using an external magnetic field, the (free) magnetization of the second permalloy layer can be manipulated relative to the (fixed) magnetization of the first permalloy layer. Since the sensor's electrical resistance is dependent on the relative orientation of the magnetizations in the two permalloy layers, the sensor can thus be used to transcribe a fluctuating external magnetic flux into a correspondingly fluctuating electrical current.
In such sensors as hereabove described, the use of FeMn as an exchange-biasing material has certain attendant disadvantages. In particular, FeMn is highly sensitive to oxidation and other corrosion, which can destroy its exchange-biasing properties, or at least cause them to radically deteriorate. This is particularly the case in so-called "sensor-in-gap" magnetic heads, which are used in hard disc memories. Although attempts can be made to protect the FeMn layer using oxidation barriers (e.g. Ta layers), such barriers tend to reduce the magnetic sensitivity of the sensor, and are seldom completely effective in the long term.
As an alternative to FeMn, the use of an exchange-biasing layer comprising nickel oxide is discussed in an article by M. J. Carey and A. E. Berkowitz in Appl. Phys. Lett. 60 (1992), pp 3060-3062. A great advantage of such an exchange-biasing material is that, since it is already oxidic, it is insensitive to further oxidation. However, a disadvantage of nickel oxide is that its maximum exchange-biasing field H.sub.eb is considerably smaller than that of FeMn. This is undesirable because, as continuing miniaturization trends demand ever smaller field sensors, demagnetization effects in such miniaturized sensors become increasingly significant, so that adequate compensation accordingly requires increasing exchange-biasing fields H.sub.eb. It should be noted that H.sub.eb is here defined as the field-axis displacement (from the zero-field line) of the hysteresis loop of an exchange-biased magnetic layer: see, for example, the article by R. Jungblut et al. in J. Appl. Phys. 75 (1994), pp 6659-6664.
In both the said article by Carey and Berkowitz and in EP 594 243, the exchange-biasing layer is provided using sputter deposition in an Ar-gas atmosphere. An advantage of sputter deposition is that it is highly compatible with large-scale, low-price industrial production, in contrast to techniques such as molecular beam epitaxy.